Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). Examples of tachyarrhythmias include supraventricular tachycardias (SVT's) such as sinus tachycardia, atrial tachycardia, and atrial fibrillation. The most dangerous tachyarrythmias, however, are ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular rhythms occur when an excitatory focus in the ventricle usurps control of the heart rate from the sinoatrial node. The result is rapid and irregular contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram because they do not use the normal ventricular conduction system, the depolarization spreading instead from the excitatory focus directly into the myocardium. Ventricular tachycardia is characterized by distorted QRS complexes occurring at a rapid rate, while ventricular fibrillation is diagnosed when the ventricle depolarizes in a chaotic fashion with no recognizable QRS complexes. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death.
Cardioversion (an electrical shock delivered to the heart synchronously with the QRS complex) and defibrillation (an electrical shock delivered without synchronization to the QRS complex to terminate ventricular fibrillation) can be used to terminate most tachycardias, including SVT's, VT, and VF. The electric shock terminates the tachycardia by depolarizing all excitable myocardium which prolongs refractoriness, interrupts reentrant circuits, discharges excitatory foci. Implantable cardioverter/defibrillators (ICD's) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device.
Another type of electrical therapy for tachycardia is antitachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt reentrant circuits causing the tachycardia. Modem ICD's have ATP capability so that ATP therapy is delivered to the heart when a tachycardia is detected, while a shock pulse is delivered when fibrillation occurs. Although cardioversion/defibrillation will terminate tachycardia, it consumes a large amount of stored power from the battery and results in some patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. Generally, only cardioversion/defibrillation will terminate fibrillation and certain high rate tachycardias, while ATP can be used to treat lower rate tachycardias.
In current ICD's with ATP capability, ventricular fibrillation (VF) is distinguished from ventricular tachycardia (VT) using rate based criteria so that ATP or shock therapy can be delivered as appropriate. The heart rate is usually measured by detection of the time between successive R waves (i.e., ventricular depolarizations). A measured heart rate is classified as a tachycardia when the rate is in a VT zone, defined as a range of rates above a tachycardia detection rate (TDR) but below a fibrillation detection rate (FDR). A measured heart rate above the FDR, on the other hand, is in the VF zone and is classified as a fibrillation.
A commonly cited figure of merit is that for a patient with a normal sinus rhythm of 70 bpm, a rate of 150 bpm is considered tachycardia and a rate of over 210 bpm, fibrillation. Normally, the ICD is programmed with a fixed tachycardia detection zone boundary (i.e., the TDR) for detection of a tachyarrhythmia, and a fixed fibrillation detection zone boundary (i.e., the FDR) to distinguish VT from VE. Making the fibrillation detection zone boundary fixed, however, has several disadvantages. First, the heart rate that constitutes fibrillation is very different from patient to patient, making it difficult to set an appropriate value at the outset. Detection zone boundaries can be determined for an individual patient by a procedure in which arrhythmias are purposely induced and then characterized based upon their rate, EKG waveforms, and response to treatment. The rate of VF and VT when induced, however, may be quite different from spontaneously occurring VT and VF. Furthermore, only a limited number of induced episodes are available to guide the programming of the detection zone boundaries, thus necessitating a larger safety margin to avoid undersensing of fibrillation because of the uncertainty as to what the true zone boundary should be. Also, certain patients may suffer from ventricular tachycardia and ventricular fibrillation which have a range of rates that overlap, making it difficult to distinguish in a single instance low rate fibrillation from high rate tachycardia. Finally, the rate which separates tachycardia from fibrillation may change over time in a given patient due to, for example, progression of a disease process or the pharmacological effects of medications.
Most ICD's activate an.electronic timer when a tachyarrhythmia is detected. If the measured heart rate is low enough that the arrhythmia is characterized as a tachycardia, ATP therapy may be delivered. Upon expiration of the timer, however, if the arrhythmia is not terminated, a shock pulse is delivered. Setting the fibrillation detection zone boundaryat an inappropriate value thus results in delayed application of a necessary defibrillation pulse if it is too high, while setting the rate too low results in needless pain being inflicted on the patient as well as wastage of limited battery power.